In communication applications, a direct conversion receiver (DCR) can be utilized to convert a modulated radio frequency (RF) signal to the original frequency or baseband frequency in a single step. Typically, the modulated RF signal undergoes a single frequency conversion to a "zero" intermediate frequency (IF) signal. The zero IF signal contains the modulated information.
Generally, a mixer is employed to generate the zero IF signal by mixing a local oscillator (LO) signal which is equal to the center or carrier frequency of the modulated RF signal. The mixed or zero IF signal contains the baseband frequency as well as the local oscillator frequency, the carrier frequency and the local oscillator frequency plus the carrier frequency (e.g., the modulation).
Since the local oscillator frequency is approximately the same as the RF frequency, the mixed signal contains the local oscillator frequency, two times the local oscillator frequency and the baseband frequency. A low-pass filter can be utilized to filter out the higher frequencies leaving only the baseband frequency. Thus, the direct conversion receiver directly detects or demodulates the RF signal and can more efficiently receive a baseband signal than super heterodyne receivers which often require several frequency conversions.
Direct conversion receivers often have a performance limitation related to even order distortion. For example, in high-performance receiver applications, standards require that a strong off-channel signal cannot mask a weak on-channel signal. Specifications usually require that the strong undesired signal can be up to 80 decibels (Db) greater than the weak desired signal, and yet the weaker desired signal must still be receivable without significant distortion. This requirement can cause problems in direct conversion receivers which are susceptible to even order distortion from off-channel signals.
Mixers, as well as other circuits in communication applications, typically have a non-linear transfer function that can be approximated by a power series. Even order distortion can generally be approximated by the squared power term or the second order distortion because it is usually the largest component. The squared term in the power series can cause the amplitude modulation (AM) of the RF signal to be directly converted to baseband (e.g., directly detected in a similar fashion to a diode detector).
This phenomena is separate from the desired frequency conversion of the mixer as the modulation of the strong signal is decoded to baseband (e.g., zero-IF) independently of the local oscillator frequency. Indeed, the local oscillator can actually be off and the non-linear nature of the mixer still decodes the amplitude modulation on strong off-carrier signals. Thus, in direct conversion receivers, the modulation on the strong off-channel signals can interfere with the conversion of the weak desired signal to baseband.
Thus, there is a need for a direct conversion receiver which is not susceptible to even order distortion performance limitations. Further still, there is a need for a mixer for use in a direct conversion receiver which can reduce even order distortion.